Broad band directional coupler



June 20, 1950 J. M. WOLF BROAD BAND DIRECTIONAL COUPLER Filed Jan. 7,1946 INVENTOR.

JA M ES M W OLF ATTORNEY.

Patented June 20, 1950 BROAD BAND DIRECTIONAL COUPLER James M. Wolf,Boston, Mass, assignor, by mesne assignments, to. the United States ofAmerica, .as represented by the Secretary of War Application January, 7,1946, Serial No. 639,642

This invention relates to an electrical apparatus and more particularlyto a microwave energy coupler.

One of the objects of the invention is to provide a coupler structure inwhich the shape and size of coupling apertures accomplishessubstantially uniform coupling over an operating frequency band. I I

Another object of the invention is to provide a microwave energy couplerin which the magnitude of coupling may be relatively large.

These and other objects and advantages of the invention will be apparentfrom the following description when read in connection with the drawing,which is an isometric view of the microwave coupler, in part cut away toindicate more clearly the coupling apertures and other internalstructure.

The present invention is related to the general class of microwavecouplers in which a length of wave guide (main guide) included in atransmission line, and another length of wave guide (secondaryguide)connected to a utilization circuit, have several-paths of energytransfer between them to achieve directionality or wave selectivity inthe coupling action. The energy transfer in prior art couplers of thisclass is relatively small in magnitude, and the amount of energytransfer varies considerably with the frequency of operation.

In accordance with the present invention, a relatively large percentageof microwave energy may be transferred from the main guide to thesecondary guide by reason of the aperture form and placement in thecomponent guides. For suitably chosen aperture shapes and sizes, theenergy transfer remains substantially constant over a relatively wideband of operation frequencies.

Referring now to the drawing, the coupler embodiment here describedincludes a main guide 5 and secondary guide 6 which are of rectangularcross section and have a narrow wall I in common. Main guide 5 may beprovided with endplates or flanges 8 and 9 as shown which adapt it to beinserted in, or connected in series with, the transmission line of amicrowave system. The narrow wall I held in common between the componentguides is provided with apertures II and [2 which are elongated in thedirection of energy transmission along the guides. The centers of theelongated apertures or slots are spaced by a quarter guide-wavelengthalong the direction of energy transmission in the guides, and the slotsare laterally spaced on each side of the longitudinal axis of secondaryguide 6. The slots 4 Claims. (Cl. 17844) may thus overlap, if desired,in the direction of energy transmission.

Each slot in the common wall 1 provides coupling between the componentwave guides of the illustrated structure. Because of the quarterguide-wavelength spacing of the coupling slots, very little wave energyis excited in secondary guide 6 in a backward direction with respect toan exciting primary wave in main guide 5. The electrical distances whichmay be traced along main guide 5, thence through the two coupling slotsin a backward direction into secondary guide 6, differs by a halfguide-wavelength. The backwardly directed components of energytransferred through slots II and I2 thus interfere destruc tively, sothat practically no energy ispropagated in a backward direction. Energypropagation in secondary guide 6 does take place in a forward direction,however, for the forwardly directed components of energy coupled intothe secondary guide are-in phase, the path lengths being equal in thatdirection. Thus, energy excited in secondary guide 6 propagates in thesame direction as the corresponding energy wave in main guide 5. v

High selectivity in coupling action is therefore achieved in thecoupling structure thus far described, for oppositely directed incidentand reflected primary waves in main guide 5 excite proportional andoppositely directed energy waves in secondary guide 6.

Referring again to the drawing, the directional coupler may be soconnected in a microwave sys-' tem that incident energy travels throughmain guide 5 in the direction indicated at l3, toward the right in thedrawing. Energy corresponding to the primary incident wave thenpropagates in the same direction in secondary guide 6. Similarly, anoppositely directed reflected wave in main guide 5 excites an energywave in secondary guide 6 which travels in the opposite direction,toward the left in the drawing. This latter energy wave, made separatelyavailable by the particular disposition of the coupling apertures as setforth, is not utilized in the instant structure and is in factdissipated as hereinafter described.

The guides may operate in the dominant (TE-0,1) mode, in which theE-vectors (electric field vectors) are normal to the broader walls. Aconventional resistive strip termination I4 is supported in an E-planeat the end of secondary guide 6 toward which the coupled reflected wavetravels. The resistive strip functions to absorb microwave energypropagated toward it and thus eliminates reflection and interference. Anenergy pick-up means at the other end is adapted to convey coupledincident energy from secondary guide 6 to an external utilizationcircuit. This The directional coupler of the present inven- .7 tion ischaracterized by the fact that it may be designed for relatively largecoupling, or for substantially constant coupling over a-broader bandthan heretofore attained, due primarily tothe elongation of the couplingapertures in the di-- rection of energy transmission in the guides. Themagnitude of coupling is determined by the size, shape, and lateralspacing of the apertures. The coupling may therefore be made as loose asdesired, for example by using slots of suitably small width. Anotherfeature of the instant direction-a1 coupler is that it may be utilizedat any location along a transmission line, for the fraction of powercoupled into the secondary guide is substantially independent of therelative phase of standing waves that may be present in the main guide.r

The invention is not limited to a directional coupler having abroad-band characteristic, for it may be desirable in certain instancesto provide other relationships of coupling to frequency, as may beaccomplished by an appropriate selection of coupling aperture shape. 7

It is apparent that the invention is not limited to the physicalconstruction illustrated in the drawing, but that various changes may bemade without departingirom the spirit of the invention.,

Wht is. claimed is: ,1 l. A directional (coupler, including a firstlength of wave guide wherein primary waves may be propagated, a secondlength of wave guide, and means interconnecting said lengths oi waveguide comprising a common wall having two spaced apertures therein, saidcommon wall being parallel to the electric field vectors in said secondlength of wave guidasaid apertures being symmetrically disposed oneither side of the longitudinal axis of said wall, being elongated inthe direction of, energy transmission and having their centers spacedalong said direction by an effective distance of a quarterguide-wavelength, whereby energy waves corresponding to and proportional to said primary waves .are propagated in the same direction insaid second length of wave guide, said directional coupler having abroad-band coupling versus frequency characteristic.

2. A directional coupler, including a first length of rectangular waveguide wherein primary waves may be propagated, a second length ofrectangular wave guide, and means interconnecting said lengths of waveguide comprising a common wall having two spaced apertures therein, saidapertures being symmetrically disposed laterally with respect to thelongitudinal axis of said wall, being elongated in the direction ofenergy transmission and having their centers spaced along said directionby an eifective distance of a quarter guide-wavelength,

-whereby relatively large energy waves corresponding to, andproportional to said primary waves are propagated in the same directionin said second length of wave guide.

3. A directional coupler, including a first length of wave guide whereinprimary waves may be propagated, a second length of wave guide, andmeans interconnecting said lengths of wave guide comprising a common.wall having two laterally disposed apertures therein, said aperturesbeing elongated in the direction of energy transmission and having theircenters spaced along said direction by'an eiiective distance of aquarter guide-wavelength, whereby energy waves corresponding to andproportional to said primary waves are propagated in the same directionin said second length of wave guide, said directional coupler having abroadband coupling versus frequency characteristic.

4. A microwave energy coupling. structure in cluding a-first length ofwave guide wherein primary waves may be propagated, a second length ofwave guide, means providing two coupling paths between said lengths ofwave guide, whereby energy waves corresponding to said primary waves arepropagated in said second length of wave guide, said means comprising apair of laterally disposed apertures elongated in the direction ofenergypropagation, having their centers spaced along said direction by aquarter guide wavelength, and means to absorb'reflected energy in saidsecond wave guide.

JAMES M. WOLF.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Great -Bri'tain June is, 1942

